A well-known method for manufacturing a polarizing film or a so-called polarizer (hereinafter, referred as “a polarizing film”) comprises steps of subjecting a film of polyvinyl alcohol resin type material (hereinafter, referred as “PVA type resin”) to dyeing and stretching processes to thereby provide a stretched film having a dichroic material impregnated therein with a molecularly oriented state.
However, it is not possible to manufacture a polarizing film having a thickness of 10 μm or less through a conventional method using a general mono-layer of PVA type resin. The reason for such difficulty is that the thickness of the mono-layer resin used in the method has to be 30 μm or less. As described later, PVA type resin is hydrophilic. The single resin layer comprised of a PVA type resin having a thickness of 30 μm or less is difficult to be processed through the dyeing and/or stretching process without being subjected to any risk of the layer being partially dissolved or broken. Specifically, there remains an unsolved technical problem of manufacturing a polarizing film having, for example, a thickness of 10 μm or less and having a uniform thickness and stable optical property.
Thus, a method for producing a polarizing film using a thermoplastic resin substrate has been proposed as shown by a schematic diagram in FIG. 24. The method for manufacturing a polarizing film using a thermoplastic resin substrate is noticeable in that it allows for making the film thinner, and thus allows for more uniform thin film compared with the method for manufacturing the polarizing film using a mono-layer PVA type resin. Nevertheless, the polarizing film having a thickness of 10 μm or less has not been in practical use. Specific examples hithertofore proposed will be described later
In the case of a polarizing film which is to be adhesively attached to each of the opposite surfaces of a liquid-crystal cell for use in a liquid-crystal display apparatus, according to the method for manufacturing a polarizing film with a mono-layer of the PVA type resin as disclosed in Japanese Patent Application Publication JP2005-266325A (the Patent Document 1), the polarizing film is manufactured, for example, by transporting a PVA type resin mono-layer of a thickness of 60 to 80 μm through a transporting apparatus comprising a plurality sets of rolls driven at different peripheral speeds, and immersing the PVA type resin mono-layer in a dyeing solution to have a dichroic material impregnated therein, while stretching the mono-layer comprising the PVA type resin in the dichroic material solution at around 60° C. This is the polarizing film manufactured from a mono-layer having a thickness of 20 to 30 μm. Presently, the polarizing film manufactured using the method has optical properties of a single layer transmittance of 42% or higher and a polarization rate of 99.95% or higher, and is used for a large-sized screen television.
However, since the PVA type resin is hydrophilic, a polarizing film is sensitive to change of temperature and humidity and is apt to produce changes in dimensions such as expansion or shrinkage due to changes in environmental conditions possibly resulting in cracks. In order to suppress expansion and shrinkage and to minimize effects of temperature and/or humidity, it has been a usual practice in a polarizing film for use with a television to provide a film of triacetylcellulose (TAC) having a thickness of 40 to 80 μm laminated on each of the opposite surfaces of the polarizing film as a protection film.
It should however be noted that, in the case of a mono-layer polarizing film, since there is a certain limit in reducing the thickness of the polarizing film, it is still difficult to completely restrict the expansion or shrinkage, so that, when an optical film laminate including such polarizing film is adhesively attached to a component such as an adjacent optical film or a liquid-crystal cell through a layer of a bonding or adhesive agent, a stress is induced in such component by the expansion or shrinkage of the polarizing film. Such stress may cause a distortion in the displayed image in a liquid-crystal display apparatus. Since the image distortion is caused by an optical elasticity or deformation of the component induced by a stress produced under the shrinkage in the polarizing film, the material to be used for such component must be limited to be of a low optical elasticity and/or of low birefringent property in order to reduce the image distortion. In addition, since the stress produced under the shrinkage of the polarizing film may possibly cause the optical film laminate to be detached from the liquid-crystal cell, it is required to use an adhesive agent of a higher adhesive power. However, using such adhesive agent of high adhesive power makes re-working operation difficult. The above is the technical problem inherent to a use of a mono-layer polarizing film.
Under such circumstances, there is a need for a manufacture of polarizing films, which can be used in place of the method using a mono-layer polarizing film in which difficulties have been encountered in an effort for producing polarizing films of a decreased thickness. However, if a PVA type resin mono-layer film having a thickness smaller than 50 μm is passed through a transporting apparatus including a plurality sets of rolls driven at different peripheral speeds and stretched in a dye solution around a temperature of 60° C. to produce a polarizing film having a thickness of 10 μm or less, the PVA type resin mono-layer comprised of a hydrophilic polymer composition may be at least partially dissolved in the solution as the thickness is decreased by stretching, or may be broken because of being unable to withstand the stretching stress.
Because of such problems, the methods disclosed in the Patent Documents 2 to 5 have been proposed as new manufacturing methods of polarizing films. In those methods, a polarizing film is manufactured by forming a layer of a PVA type resin on a thermoplastic resin substrate of a certain thickness and stretching thus formed PVA type resin layer together with the thermoplastic resin substrate. Specifically, the methods correspond to reference test samples 1, 2 or 3. Describing in more detail, the method corresponding to the reference test samples 1 or 2 includes steps of providing a laminate comprising a thermoplastic resin substrate and a PVA type resin layer and subjecting the laminate to stretching in air typically at a stretching temperature of 60 to 110° C. using, for example, a stretching apparatus arranged in an oven. Then, the PVA type resin layer having molecular orientation produced through the stretching is subjected to a dyeing process to have a dichroic material impregnated therein. According to the method corresponding to the reference test sample 3, the PVA type resin layer in the laminate comprising the thermoplastic resin substrate and the PVA type resin layer is first dyed to have a dichroic material impregnated therein. Then, the laminate comprising the PVA type resin layer having the dichroic material impregnated therein is stretched in air typically at a stretching temperature of 60 to 110° C. The above process is the method of manufacturing the polarizing film having the dichroic material in a molecularly oriented state as disclosed in the Patent Documents 2 to 5. These are the methods which have hithertofore been proposed for manufacturing a polarizing film using a thermoplastic resin substrate. FIG. 25 shows results of verifying test performed by the inventors for determining optical properties accomplished in the polarizing film disclosed in the Patent Document 2 to 5.
The polarizing film produced by the aforementioned manufacturing process is promising from viewpoint of reducing the thickness of a large size display element, eliminating distortions in displayed images and reducing industrial waste. However, up to now, the polarizing film manufactured by the aforementioned process has not been practically used. The reason for this is considered as that the polarizing film has a lower degree of optical properties in terms of the polarizing property, as shown in FIG. 25 illustrating the optical properties of the reference test samples 1 to 3, and thus, there is still unsolved technical problem in an effort for producing a polarizing film with a sufficiently high optical property.
The prior art documents referred to in the above and following descriptions are listed below.                Patent Document 1: Japanese Laid-Open Patent Publication JP2005-266325A        Patent Document 2: Japanese Patent 4279944B        Patent Document 3: Japanese Laid-Open Patent Publication JP2001-343521A        Patent Document 4: Japanese Patent Publication JP8-12296B        Patent Document 5: U.S. Pat. No. 4,659,523        Non-Patent Document 1: H, W. Siesler, Adv. Polym. Sci., 65, 1 (1984)        